1. Field of the Invention
The present invention relates to a multi-spindle machining apparatus for and a multi-spindle machining method of finishing internal cylindrical surfaces of workpieces such as connecting rods, for example, which are employed in engines for motor vehicles.
2. Description of Related Art
Hitherto, there is known such a type of multi-spindle machine tool that, when machining internal cylindrical surfaces of openings of the connecting rods for use in the motor vehicle engines, for example, it carries out machining works of such internal cylindrical surfaces of the plural connecting rods simultaneously by each cutting edge provided on a plurality of machining spindles which are rotated together through a common driving source. With these multi-spindle machines, firstly a rough broaching or a rough boring, then a finish-boring and lastly an internal honing as an super-finishing process, for example, are in turn taken place simultaneously with respect to the plurality of connecting rods.
By the way, a large end of the connecting rod is divided into a cap portion and a body portion and hence formed with discontinuities at joint portions s,s between the cap portion and the body portion, as seen in FIG. 6. The discontinuity is also formed by an oil hole h open to the internal cylindrical surface of the large end. The reason why the super-finish honing as described above is inevitably applied to the connecting rods resides in that due to the discontinuities a predetermined surface roughness and circularity of the internal machining surface of the large end may not be obtained through the finishing process, as seen in FIG. 6(A). Namely, when the cutting edge passes the joint portions s,s and the oil hole h, the chattering or oscillation is susceptible to being developed, and when the plurality of connecting rods are fixed in position in the same posture and machined by the cutting edges of the same phase of rotation, the chattering or the like is amplified, since the plurality of the cutting edges pass almost at the same time the discontinuous portions.
When shifting the phases of rotation of the cutting edges with respect to each of the connecting rods so as to adjust the timings, at which the cutting edges pass the discontinuities, to be differed from each other, the petalous swells are susceptible to being developed periodically or cyclically along the circumferential direction, as illustrated in FIG. 6(B), and the improved effect is not obtained so much. Thus, it is desired to provide such a machining art that the predetermined surface roughness and circularity are capable of being obtained without super-finishing, in the case where the plurality of workpieces with a discontinuity on the surface to be machined are simultaneously machined by the multi-spindle machine tool.